1. Field of the Invention
The present invention relates generally to surgical implants and, more particularly, to surgical implants of the modular type and formed of titanium and its alloys, and a process of improving their desirable characteristics by ion implanting the same.
2. The Prior Art
Titanium-based alloys have come to the fore of late in replacing cobalt-based alloys that had been traditionally used as surgical implants. A number of reasons are responsible for the switch to titanium-based alloys. These include: excellent tensile strength, high fatigue strength, low density, high corrosion resistance, substantial ductility, a low modulus of elasticity compatible with bone structure that facilitates good adhesion thereto and, most importantly, excellent biocompatibility. One questionable property of titanium-based alloys has proven to be their wear resistance. The wear performance of surgical implants made from titanium-based alloys already has been improved upon by ion implantation, in particular by implantation of carbon and nitrogen ions directly into the surface of the surgical implants. See "Ion Beam Modification of Materials for Industry," Thin Solid Films, 118 (1984) 61-71; "The Wear Behavior of Nitrogen-Implanted Metals," Metallurgical Transactions, A 15 (1984), 2221-2229; and "Wear improvement of surgical titanium alloys by ion implantation;" J. Vac. Sci. Tech.A3 (6) November/December 1985, 2670-2674. See also U.S. Pat. No. 4,693,760 granted on Sep. 15, 1987 to Piran Sioshansi, one of the co-inventors herein, and assigned to a common assignee, Spire Corporation of Bedford, Mass., the disclosure of which is incorporated herein by reference.
Titanium and its alloys are thus the most widely used alloys for securing surgical and dental implants against bone. Initially, most if not all surgical implants were secured to bone using polymethylmethacrylate (PMMA) cement for such implant to be considered as properly secured in place. When using PMMA cement, there is very little if any movement of the implant and there is but minimal contact thereof with the adjacent bone structure. In implants that are loosened over time however, there develops a micromotion between the implant and the bone or between the implant and the cement interface. Once such micromotion i.e., vibration, has developed, blackened tissue has been observed in the affected area. Micromotion of course also is present in cementless fixation of implants. Cementless fixation relies on securing the implant in and to the surrounding bone structure with frictional contact, with or without the aid of auxiliary fixation devices implanted adjacent to and concurrently with the surgical implant. The observed blackened tissue is caused by fretting wear and abrasion of the titanium alloy against the surrounding bone structure. This fretting wear and resulting blackening of tissue is and remains a vexing problem in bone and dental surgery that must be addressed and hopefully solved if present day fixation methods for such implants are to receive favorable widespread acceptance.